BFM-TV reported he was a 70-year-old German, a monitor at one campsite who was allegedly swept away inside his van.
The mayor of Bagnols, Jean-Yves Chapelet, told BFM-TV many Dutch tourists were among the 350 campers evacuated there.
Trapped by a fast-flowing torrent, he sought shelter inside his caravan - but the vehicle was swept away, the AFP news agency reports.
Hundreds of rescuers backed by helicopters moved out about 1,600 people, majority campers, in three regions of southern France where heavy rain caused flash flooding and transformed rivers and streams into torrents, the interior minister said Thursday. Everything is being done to try and find him, with waterways being searched where possible.
In the Gard region, where strong rains poured down all day on Thursday, rivers quickly overflowed their banks and 119 children were rescued from their campsite at Saint-Julien-de-Peyrolas, the French interior minister said in a statement.
The worst-hit areas are Gard, Ardèche and Drôme.
The spokesman said: "Like everybody else, they should remain on their guard".
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"No one has suitcases".
Hundreds of firefighters and police took part in the rescue operations, using helicopters to help spot affected campsites and carry out some people.
Nearly 75mm of rain fell over the area in just 24 hours with more outbreaks possible during the next few days.
Several parts of central and southeast France are on flood alert after a blazing heatwave ended abruptly in storms on Thursday.
Heavy rain meant tents were destroyed, while about 17,000 homes were without power.
Electricity supplies have been cut in some villages, while roads have been gridlocked because of the number of people trying to get to high ground.